1. Field of the Invention
The present invention relates to an organic light-emitting device in which at least two hole transport materials are used as phosphorescent hosts, thereby improving the efficiency and the lifespan of the device.
2. Description of the Background
Light-emitting materials that are used in organic light-emitting devices are classified into fluorescent materials that use singlet excitons and phosphorescent materials that use triplet excitons, which differ in their emission mechanisms.
Generally, a phosphorescent material comprises an organic metal compound that includes heavy atoms. In a phosphorescent material, an exciton transitions from a triplet state to a singlet state and emits light as a result. The phosphorescent material may use the triplet excitons, which make up 75% of the excitons, and therefore has much higher emission efficiency than the fluorescent material that uses singlet excitons, which make up the remaining 25% of excitons.
A light-emitting layer comprising a phosphorescent material includes host material and a dopant material that receives energy from the phosphorescent host material to emit light. Several phosphorescent dopant materials using an iridium metal compound have been reported by Princeton University and the University of Southern California. Specifically, (4,6-F2ppy)2Irpic and an iridium compound based on a fluorinated ppy ligand structure have been developed as blue light-emitting materials. Their host material is typically 4,4′-biscarbazolylbiphenyl (CBP). It has been reported that a triplet state energy band gap of a CBP molecule is appropriate for producing green and red light, but since the energy band gap of a CBP molecule is less than the energy gap of a blue material, a very inefficient endothermic energy transition may be required to produce blue light. As a result, a CBP host causes a blue light-emitting material to have low emission efficiency and a short lifespan.
Recently, a carbazole-based compound that has a larger triplet energy band gap than CBP has been used when forming the light-emitting layer with a phosphorescent material.
However, when a conventional carbazole-based compound is used, a phosphorescence device may be inefficient and have a short lifespan, and may have much room for improvement.